Implantable extravascular electrical stimulation lead having improved sensing and pacing capability

ABSTRACT

Implantable medical electrical leads having electrodes arranged such that a defibrillation coil electrode and a pace/sense electrode(s) are concurrently positioned substantially over the ventricle when implanted as described. The leads include an elongated lead body having a distal portion and a proximal end, a connector at the proximal end of the lead body, a defibrillation electrode located along the distal portion of the lead body, wherein the defibrillation electrode includes a first electrode segment and a second electrode segment proximal to the first electrode segment by a distance. The leads may include at least one pace/sense electrode, which in some instances, is located between the first defibrillation electrode segment and the second defibrillation electrode segment.

This application is a continuation of U.S. patent application Ser. No.14/695,255 filed Apr. 24, 2015, which claims the benefit of U.S.Provisional Application No. 61/984,148 filed on Apr. 25, 2014, theentire content of both of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present application relates to electrical stimulation leads and,more particularly, electrical stimulation leads with improved sensingand/or pacing capability for use in extravascular applications.

BACKGROUND OF THE INVENTION

Malignant tachyarrhythmia, for example, ventricular fibrillation (VF),is an uncoordinated contraction of the cardiac muscle of the ventriclesin the heart, and is the most commonly identified arrhythmia in cardiacarrest patients. If this arrhythmia continues for more than a fewseconds, it may result in cardiogenic shock and cessation of effectiveblood circulation. As a consequence, sudden cardiac death (SCD) mayresult in a matter of minutes.

In patients at high risk of ventricular fibrillation, the use of animplantable cardioverter-defibrillator (ICD) system has been shown to bebeneficial at preventing SCD. An ICD system includes an ICD, which is abattery powered electrical stimulation device, that may include anelectrical housing electrode (sometimes referred to as a can electrode),that is coupled to one or more electrical stimulation leads. Theelectrical stimulation leads may be placed within the heart, withinvasculature near the heart (e.g., within the coronary sinus), attachedto the outside surface of the heart (e.g., in the pericardium orepicardium), or implanted subcutaneously above the ribcage/sternum. Ifan arrhythmia is detected, the ICD may generate and deliver a pulse(e.g., cardioversion or defibrillation shock) via the electricalstimulation leads to shock the heart and restore its normal rhythm.

SUMMARY

Extravascular and/or extracardiac implanted electrical stimulationleads, e.g., subcutaneously implanted electrical stimulation leads orsubsternally implanted electrical stimulation leads reside in a plane oftissue or muscle between the skin and sternum for subcutaneous, orreside in a plane of tissue or muscle between the sternum and the heartfor substernal. Due to the distance between the heart and electrodes ofthe electrical stimulation leads, to achieve improved pacing, sensing,and/or defibrillation, the pace/sense electrodes and the defibrillationcoil electrode should be positioned in the plane of tissue such that theelectrodes are located directly above or proximate the surface of thecardiac silhouette, most typically the ventricular surface. For example,the electrode(s) used to deliver pacing pulses should be positioned in avector substantially over (and in some instances centered over) thechamber to be paced to produce the lowest pacing capture thresholds forpacing. Likewise, the electrode(s) used to sense cardiac electricalactivity of the heart should be positioned substantially over (and insome instances centered over) the chamber to be sensed to obtain thebest sensed signal. For shocking purposes, it is preferred to have thedefibrillation coil electrode positioned substantially over (and in someinstances centered over) the center of the chamber to be shocked.

Current medical electrical lead designs used for subcutaneousdefibrillation include a single defibrillation coil electrode locatedbetween a first pace/sense electrode distal to the defibrillation coiland a second pace/sense electrode proximal to the defibrillation coil.In such a configuration, it is not possible to concurrently positionboth the defibrillation coil electrode and the pace/sense electrode(s)substantially over the center of the ventricle. Electrical stimulationleads described herein are designed such that concurrent positioning ofthe defibrillation electrode and the pace/sense electrode(s) ispossible.

In one example, this disclosure is directed to an implantable medicalelectrical lead comprising an elongated lead body having a distalportion and a proximal end, a connector at the proximal end of the leadbody, and a defibrillation electrode located along the distal portion ofthe lead body. The defibrillation electrode includes a first electrodesegment and a second electrode segment. The second electrode segment isspaced proximal to the first electrode segment by a distance. The leadalso includes at least one pace/sense electrode located between thefirst segment and the second segment of the defibrillation electrode.

In another example, this disclosure is directed to an implantablemedical electrical lead an elongated lead body having a distal portionand a proximal end, a connector at the proximal end of the lead body,and a defibrillation electrode located along the distal portion of thelead body. The defibrillation electrode includes a first electrodesegment and a second electrode segment spaced proximal to the firstelectrode segment by approximately 1-3 centimeters (cm). The lead alsoincludes at least one pace/sense electrode located between the firstelectrode segment and the second electrode segment of the defibrillationelectrode.

In a further example, this disclosure is directed to an extravascularimplantable cardioverter-defibrillator system comprising an implantablecardioverter-device (ICD) that includes a therapy module configured togenerate and deliver electrical stimulation therapy and an extravascularimplantable medical electrical lead electrically coupled to the therapymodule. The lead includes an elongated lead body having a distal portionand a proximal end, a connector at the proximal end of the lead body,and a defibrillation electrode located along the distal portion of thelead body. The defibrillation electrode includes a first electrodesegment and a second electrode segment spaced proximal to the firstelectrode segment by a distance. The lead also includes at least onepace/sense electrode located between the first electrode segment and thesecond electrode segment of the defibrillation electrode.

In a further example, this disclosure is directed to an extravascularimplantable cardioverter-defibrillator (ICD) system comprising anextravascular implantable medical electrical lead and an ICD coupled tothe extravascular lead. The extravascular lead includes an elongatedlead body having a distal portion and a proximal end, a connector at theproximal end of the lead body, and a plurality of defibrillationelectrode segments located along the distal portion of the lead body.The plurality of defibrillation electrode segments include at least afirst defibrillation electrode segment and a second defibrillationelectrode segment spaced proximal to the first defibrillation electrodesegment by a distance. The extravascular lead further includes a firstpace/sense electrode located distal to the first defibrillationelectrode segment and a second pace/sense electrode located proximal tothe second defibrillation electrode segment. The extravascular leadfurther includes a plurality of conductors extending within theelongated body from the connector to the distal portion, wherein each ofthe first defibrillation electrode segment, the second defibrillationelectrode segment, the first pace/sense electrode, and the secondpace/sense electrode are coupled to a different one of the plurality ofconductors within the lead body. The ICD includes a therapy moduleconfigured to generate and deliver electrical stimulation therapy and aswitch module configured to selectively couple the therapy module to anelectrode vector in which both the first and second defibrillationelectrode segments simultaneously function as a cathode for delivery ofdefibrillation therapy.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual drawings illustrating various views of apatient implanted with an example extravascular implantablecardioverter-defibrillator (ICD) system.

FIGS. 2A-2C are conceptual drawings illustrating various views of apatient implanted with an example extravascular ICD system in which adistal portion of the lead is implanted substernally.

FIG. 3 is a drawing illustrating a distal portion of an exampleextravascular implantable medical electrical lead.

FIG. 4 is a drawing illustrating a distal portion of another exampleextravascular implantable medical electrical lead.

FIG. 5 is a drawing illustrating a distal portion of another exampleextravascular implantable medical electrical lead.

FIG. 6 is a block diagram illustrating components of an example ICD.

DETAILED DESCRIPTION

FIGS. 1A and 1B are conceptual diagrams of an extravascular and/orextracardiac implantable cardioverter-defibrillator (ICD) system 10. ICDsystem 10 is subcutaneously implanted within a patient 12. FIG. 1A is afront view of ICD system 10 implanted within patient 12. FIG. 1B is aside view of ICD system 10 implanted within patient 12. ICD system 10includes an ICD 14 connected to an extravascular and/or extracardiacelectrical stimulation lead 16. FIGS. 1A and 1B are described in thecontext of an ICD system capable of providing defibrillation and/orcardioversion shocks and, in some instances, pacing pulses. However, thetechniques of this disclosure may also be used in the context of otherimplantable medical devices configured to provide electrical stimulationpulses to stimulate other portions of the body of patient 12.

ICD 14 may include a housing that forms a hermetic seal that protectscomponents of ICD 14. The housing of ICD 14 may be formed of aconductive material, such as titanium or titanium alloy. The housing ofICD 14 may function as a housing electrode (sometimes referred to as acan electrode). In other instances, the housing of ICD 14 may include aplurality of electrodes on an outer portion of the housing. The canelectrode and/or the plurality of electrodes on the outer portion of thehousing may be coated with a material, such as titanium nitride. ICD 14may also include a connector assembly (also referred to as a connectorblock or header) that includes electrical feedthroughs through whichelectrical connections are made between conductors extending within thebody of lead 16 and electronic components included within the housing ofICD 14. As will be described in further detail herein, housing may houseone or more processors, memories, transmitters, receivers, sensors,sensing circuitry, therapy circuitry, power sources and otherappropriate components. The housing is configured to be implanted in apatient, such as patient 12. ICD 14 is implanted subcutaneously on theleft side of patient 12 above the ribcage. ICD 14 may, in someinstances, be implanted between the left posterior axillary line and theleft anterior axillary line of patient 12. ICD 14 may, however, beimplanted at other subcutaneous locations on patient 12 as describedlater.

Lead 16 includes an elongated lead body having a proximal end thatincludes a connector (not shown) configured to be connected to ICD 14and a distal portion that includes one or more electrodes. In theexample illustrated in FIGS. 1A and 1B, the distal portion of lead 16includes defibrillation electrode segments 24A and 24B, and pace/senseelectrodes 28A, 28B, and 30. In some cases, defibrillation electrodesegments 24A and 24B may together form a defibrillation electrode inthat they are configured to be activated concurrently. Alternatively,defibrillation electrode segments 24A and 24B may form separatedefibrillation electrodes in which case each of the electrodes 24A and24B may be activated independently. In some instances, ICD 14 mayinclude switching mechanisms to allow defibrillation electrode segments24A and 24B coupled to separate conductors to be utilized as a singledefibrillation electrode (e.g., activated concurrently to form a commoncathode or anode) or as separate defibrillation electrodes, (e.g.,activated individually). Electrode segments 24A and 24B are referred toas defibrillation electrode segments or defibrillation electrodesbecause they are utilized, individually or collectively, for deliveringhigh voltage stimulation therapy (e.g., cardioversion or defibrillationshocks). However, electrode segments 24A and 24B may also be utilized toprovide pacing functionality, sensing functionality or both pacing andsensing functionality in addition to or instead of high voltagestimulation therapy. In this sense, the use of the terms “defibrillationelectrode segments” or “defibrillation electrode” should not beconsidered as limiting the electrode segments to use in only highvoltage applications.

Lead 16 extends subcutaneously or submuscularly above the ribcage fromthe connector block of ICD 14 toward a center of the torso of patient12, e.g., toward xiphoid process 20 of patient 12. At a location nearxiphoid process 20, lead 16 bends or turns and extends superiorsubcutaneously above the ribcage and/or sternum, substantially parallelto sternum 22. Although illustrated in FIGS. 1A and 1B as being offsetlaterally from and extending substantially parallel to sternum 22, lead16 may be implanted at other locations, such as over sternum 22, offsetto the right or left of sternum 22, angled lateral from sternum 22 ateither the proximal or distal end, or the like. Alternatively, lead 16may be placed along other subcutaneous paths. The path of lead 16 maydepend on the location of ICD 14 or other factors.

The elongated lead body of lead 16 contains a plurality of electricalconductors (not illustrated) that extend within the lead body from theconnector at the proximal lead end to defibrillation electrode segments24A and 24B and pace/sense electrodes 28A, 28B, and 30 located along thedistal portion of the lead body of lead 16. The elongated lead body mayhave a generally uniform shape along the length of the lead body. In oneexample, the elongated lead body may have a generally tubular orcylindrical shape along the length of the lead body. The elongated leadbody may have a diameter of between 3 and 9 French (Fr) in someinstances. However, lead bodies of less than 3 Fr and more than 9 Fr mayalso be utilized. In another example, the distal portion (or all of) theelongated lead body may have a flat, ribbon or paddle shape. In thisinstance, the width across the flat portion of the flat, ribbon orpaddle shape may be between 1 and 3.5 mm. Other lead body designs may beused without departing from the scope of this disclosure. The lead bodyof lead 16 may be formed from a non-conductive material, includingsilicone, polyurethane, fluoropolymers, mixtures thereof, and otherappropriate materials, and shaped to form one or more lumens withinwhich the one or more conductors extend. However, the techniques are notlimited to such constructions.

The one or more elongated electrical conductors contained within thelead body of lead 16 may engage with respective defibrillation electrodesegments 24A and 24B and pace/sense electrodes 28A, 28B, and 30. In oneexample, each of electrodes 28A, 28B, and 30 are electrically coupled toa separate respective conductor within the lead body. Defibrillationelectrode segments 24A and 24B may be electrically coupled to a commonconductor or to separate conductors.

In one example, defibrillation electrode segments 24A and 24B may beelectrically coupled to the same conductor. For example, a single wireconductor (not shown) may be disposed within a lumen of the lead bodythat is electrically coupled to the connector at the proximal end of thelead body. The single conductor may branch off in the lead body at apredetermined longitudinal position into two or more wire conductors toconnect to each of the electrode segments 24A and 24B, respectively.Alternatively, a first conductor may connect to electrode segment 24Band a second conductor may electrically connect defibrillation electrodesegment 24B to defibrillation electrode segment 24A such thatapplication of a voltage/current to the first conductor applies thevoltage to both defibrillation electrode segment 24A and 24B. In thismanner, defibrillation electrode segments 24A and 24B are electricallycoupled such that they simultaneously function as a common anode orcathode of an electrode vector.

In other configurations, the defibrillation electrode segments 24A and24B may be coupled to separate conductors within the lead body 12. Forexample, a first electrical conductor disposed within the elongate leadbody may have a distal end coupled to defibrillation electrode segment24A and a proximal end coupled to the connector of lead 16 and a secondelectrical conductor disposed within the elongate lead body has a distalend coupled to the second defibrillation electrode segment 24B and aproximal end coupled to the connector. In this case, each of thedefibrillation electrode segments 24A or 24B may be independentlyutilized as part of an electrode vector. Additionally, ICD 14 mayinclude a switch module that may enable the conductors of bothdefibrillation electrode segments to be jumpered, tied or otherwiseelectrically connected (e.g., within the connector block and/or withinIMD 14 and/or via an external source of stimulation) such thatdefibrillation electrode segments 24A and 24B may be electricallycoupled together to be simultaneously used as a common anode or cathodeof an electrode vector for delivery of electrical stimulation therapy topatient 12 and/or for sensing the electrical signals of the heart of thepatient 12.

In any case, the respective conductors may electrically couple tocircuitry, such as a therapy module or a sensing module, of ICD 14 viaconnections in the connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of defibrillation electrode segments24A and 24B and/or pace/sense electrodes 28A, 28B, and 30 and transmitsensed electrical signals from one or more of defibrillation electrodesegments 24A and 24B and/or pace/sense electrodes 28A, 28B, and 30 tothe sensing module within ICD 14.

Defibrillation electrode segments 24 are located along the distalportion of defibrillation lead 16, e.g., toward the portion ofdefibrillation lead 16 extending superior near sternum 22. As indicatedabove, a defibrillation electrode may be formed of a first electrodesegment 24A and a second electrode segment 24B proximal to electrodesegment 24A. Electrode segments 24A and 24B are separated by a distance.In one example, the first defibrillation electrode segment 24A may bespaced apart from the second defibrillation electrode segment 24B byapproximately 1 mm-3 cm. In another example, the first defibrillationelectrode segment 24A may be spaced apart from the second defibrillationelectrode segment 24B by approximately 0.5-2 cm. In a further example,the first defibrillation electrode segment 24A may be spaced apart fromthe second defibrillation electrode segment 24B by approximately 0.5-1cm. In one example, first segment 24A and second segment 24B are eachapproximately 2-5 cm in length and the proximal end of segment 24A isseparated by approximately 1-3 cm from the distal end of segment 24B.

Defibrillation electrode segments 24 may, in one example, be coilelectrode segments disposed in-line with, around the exterior of orwithin the wall of the lead body of lead 16. In other embodiments,however, defibrillation electrode segments 24 may be a flat ribbonelectrode, paddle electrode, braided or woven electrode, mesh electrode,directional electrode, patch electrode or other type of electrode thatis segmented in the manner described herein. Moreover, in otherexamples, lead 16 may include more than two defibrillation electrodesegments. Further, the two or more defibrillation electrode segments 24may be of the same or different sizes, shapes, types or materials.

Each of first defibrillation electrode segment 24A and seconddefibrillation electrode segment 24B may be approximately 1-10 cm inlength and, more preferably, 2-6 cm in length and, even more preferably,3-5 cm in length. However, lengths of greater than 10 cm and less than 1cm may be utilized without departing from the scope of this disclosure.A total length of defibrillation electrode segments 24 may varydepending on a number of variables. The defibrillation electrode may, inone example, have a total length (e.g., length of the two segments 24Aand 24B combined) of between approximately 5-10 centimeters (cm).However, the defibrillation electrode segments 24 may have a totallength less than 5 cm and greater than 10 cm in other embodiments. Inanother example, defibrillation electrode segments 24 may have a totallength of approximately 2-16 cm. In some instances, defibrillationsegments 24A and 24B may be approximately the same length. In otherinstances, one of defibrillation segments 24A and 24B may be longer orshorter than the other one of the defibrillation segments 24A and 24B.

Defibrillation lead 16 also includes pace/sense electrodes 28A, 28B, and30 located along the distal portion of defibrillation lead 16. In theexample illustrated in FIGS. 1A and 1B, electrodes 28A and 28B arelocated between defibrillation electrode segments 24A and 24B andelectrode 30 is located distal to defibrillation electrode segment 24A.Electrodes 28A, 28B and 30 are referred to as pace/sense electrodesbecause they are generally configured for use in low voltageapplications, e.g., used as either a cathode or anode for delivery ofpacing pulses and/or sensing of cardiac electrical signals. In someinstances, electrodes 28A, 28B, and 30 may provide only pacingfunctionality, only sensing functionality or both and, in someinstances, may even be used as part of an electrode vector for highvoltage therapy, e.g., defibrillation or cardioversion shocks.

In the example of FIGS. 1A and 1B, electrodes 28A and 28B areillustrated as ring electrodes and electrode 30 is illustrated as ahemispherical tip electrode. However, electrodes 28A, 28B, and 30 maycomprise any of a number of different types of electrodes, includingring electrodes, short coil electrodes, paddle electrodes, hemisphericalelectrodes, directional electrodes, segmented electrodes, or the like,and may be positioned at any position along the distal portion of lead16. Further, electrodes 28A, 28B, and 30 may be of similar type, shape,size and material or may differ from each other. In another embodiment,for example, electrode 30 may not be a hemispherical tip electrode butinstead may be located proximal to the distal end of the lead body oflead 16 and distal defibrillation electrode segment 24A. As anotherexample, electrodes 28A and 28B may be formed of a conductive materialthat only extends around a portion of the circumference of the leadbody, e.g., a half-ring electrode, quarter-ring electrode, or otherpartial-ring electrode. In another example, electrodes 28A and 28B maybe formed of conductive material that extends around the entirecircumference of the lead body, but may be partially coated with aninsulating material to form the half-ring electrode, quarter-ringelectrode, or other partial-ring electrode. Likewise, electrode 30 maybe formed into a partial-hemispherical electrode in a similar manner asdescribed above with respect to ring electrodes 28A and 28B. In stillother instances, one or more of electrodes 28A, 28B, and 30 may besegmented electrodes (e.g., half- or quarter-ring or hemisphericalelectrodes) with separate conductors connected to each of the segmentsor a single conductor with a multiplexor or other switch to switchbetween the segmented electrodes such that the segments may be used asindividual electrodes.

Electrodes 28A, 28B, and 30 of lead 16 may have substantially the sameouter diameter as the lead body. In one example, electrodes 28A, 28B,and 30 may have surface areas between 1.6-150 mm². Electrodes 28A, 28B,and 30 may, in some instances, have substantially the same surface areaor different surface areas.

Electrodes 28A, 28B, and 30 may be spaced apart from the respectivedefibrillation electrode segments 24A and 24B by a distance greater thanor equal to 2 mm. In some instances, the distance between the closestelectrode segment 24A and 24B and electrodes 28A, 28B, and 30 is greaterthan or equal to 2 mm and less than or equal to 1.5 cm. In anotherexample, electrodes 28A, 28B, and 30 may be spaced apart from theclosest one of electrode segments 24A and 24B by greater than or equalto 5 mm and less than or equal to 1 cm. In a further example, electrodes28A, 28B, and 30 may be spaced apart from the closest one of electrodesegments 24A and 24B by greater than or equal to 6 mm and less than orequal to 8 mm. In another example, electrode 30 (or 28A and 28B) isspaced apart from the distal end of defibrillation electrode segment 24Aby a distance, which is less than or equal to 2 cm. The spacing betweeneach of electrodes 28A, 28B, and 30 and the closest one of electrodesegments 24A and 24B may be substantially the same or different.However, electrodes 28A, 28B, and 30 may be spaced apart from the distalor proximal end(s) of defibrillation electrode segment 24A or 24B byother distances without departing from the scope of this disclosure.

In some instances, the distal portion of lead 16 from the distal end oflead 16 to the proximal side of the most proximal electrode (e.g.,electrode segment 24B in the example of FIGS. 1A and 1B) may be lessthan or equal 15 cm and, more preferably, less than or equal to 13 cmand even more preferably less than or equal to 10 cm.

Electrodes 28A and 28B are spaced apart from one another along thelength of lead 16. The spacing between electrodes 28A and 28B may bedependent upon the configuration of lead 16. For example, the spacingbetween electrodes 28A and 28B is dependent upon the distance betweendefibrillation electrode segments 24A and 24B. In one example,electrodes 28A and 28B are spaced apart by less than 2 cm. In someinstances, electrodes 28A and 28B may be spaced apart by less than 1 cm.In further instances, electrodes 28A and 28B may be spaced apart fromone another by more than 2 cm.

The example configuration of electrodes and dimensions provided aboveare exemplary in nature and should not be considered limiting of theembodiments described herein. In other embodiments, lead 16 may includeless than three pace/sense electrodes or more than three pace/senseelectrodes. In further instances, the pace/sense electrodes 28A, 28B,and 30 may be located elsewhere along the length of lead 16, e.g.,distal to defibrillation electrode segment 24A, proximal todefibrillation electrode segment 24B, and/or between segments 24A and24B. For example, lead 16 may include a single pace/sense electrode 28between defibrillation electrode segments 24A and 24B and no pace/senseelectrode distal of defibrillation electrode segment 24A or proximaldefibrillation electrode segments 24B. In other examples, lead 16 mayinclude only a single pace/sense electrode 28 between defibrillationelectrode segments 24A and 24B and include another discrete electrode(s)distal to defibrillation electrode segment 24A and/or proximal todefibrillation electrode segments 24B. In still other instances, theremay be no discrete pace/sense electrodes, in which case thedefibrillation electrode segments 24A and 24B would be utilized forpacing and/or sensing. In other examples, lead 16 may include more thantwo defibrillation electrode segments 24, such as three segments withelectrode 28B located between the proximal segment and middle segmentand electrode 28A located between the middle segment and the distalsegment. Any of these multiple defibrillation electrode segments may beon a single conductor (i.e., all segments electrically coupled to asingle conductor extending within the lead body to the connector),individual conductors (i.e., each of the defibrillation electrodesegments is electrically coupled to separate conductors extending withinthe lead body to the connector), or combinations thereof (some segmentscoupled together to a common conductor and others to individualconductors). Moreover, lead 16 may include any number of pace/senseelectrodes proximal to, distal to, or between any of the multipledefibrillation electrode segments.

To achieve improved sensing and/or pacing, it is desirable to have thepace/sense electrodes located substantially over the chamber of heart 26that is being paced and/or sensed. For example, it is desirable tolocate the pace/sense electrodes over a cardiac silhouette of theventricle as observed via an anterior-posterior (AP) fluoroscopic viewof heart 26 for sensing and/or pacing the ventricle. Likewise, toachieve improved defibrillation therapy, it is desirable to have thedefibrillation electrode segments located substantially over the chamberof heart 26 to which the defibrillation or cardioversion shock is beingapplied, e.g., over a cardiac silhouette of the ventricle as observedvia an AP fluoroscopic view of heart 26. In conventional subcutaneouslead designs, it is only possible to position either the defibrillationelectrode or the sense electrode over the relevant chamber, but notboth.

Leads designed in accordance with any of the techniques described hereincan be implanted to achieve desirable electrode positioning for bothdefibrillation and pacing/sensing. In particular, lead 16 may beimplanted such that electrodes 28A and 28B are substantially locatedover a cardiac silhouette of the ventricle as observed via an APfluoroscopic view of heart 26. In other words, lead 16 may be implantedsuch that one or both of a unipolar pacing/sensing vector from electrode28A and 28B to a housing electrode of ICD 14 are substantially acrossthe ventricle(s) of heart 26. The therapy vector may be viewed as a linethat extends from a point on electrode 28A and 28B, e.g., center ofelectrode 28A and 28B, to a point on the housing electrode of ICD 14,e.g., center of the housing electrode. In another example, the spacingbetween electrodes 28A and 28B as well as the placement of lead 16 maybe such that a bipolar pacing vector between electrode 28A and electrode28B is centered or otherwise located substantially over the ventricle.

Electrode 30 may be located over the cardiac silhouette of the atrium ornear the top of the cardiac silhouette of the atrium or ventricle asobserved via an AP fluoroscopic view. As such, electrode 30 may offer analternate sensing vector and/or provide atrial pacing if needed ordesired. Thus, in some instances, lead 16 may be utilized for dualchamber pacing.

Not only are electrodes 28A and 28B located over the ventricle, butdefibrillation electrode segments 24A and 24B are substantially over(e.g., centered or otherwise) the cardiac silhouette of the ventricle asobserved via an AP fluoroscopic view of heart 26. As such, the therapyvector from defibrillation electrode segments 24A and 24B to the housingof ICD 14 is substantially across the ventricles of heart 26.

In some instances, electrode segments 24 and/or electrodes 28A, 28B,and/or 30 of lead 16 may be shaped, oriented, designed or otherwiseconfigured to reduce extra-cardiac stimulation. For example, electrodesegments 24 and/or electrodes 28A, 28B, and/or 30 of lead 16 may beshaped, oriented, designed, partially insulated or otherwise configuredto focus, direct or point electrode segments 24 and/or electrodes 28A,28B, and/or 30 toward heart 26. In this manner, pacing pulses deliveredvia lead 16 are directed toward heart 26 and not outward toward skeletalmuscle. For example, electrode segments 24 and/or electrodes 28A, 28B,and/or 30 of lead 16 may be partially coated or masked with a polymer(e.g., polyurethane) or another coating material (e.g., tantalumpentoxide) on one side or in different regions so as to direct thepacing signal toward heart 26 and not outward toward skeletal muscle. Inthe case of a ring electrode, for example, the ring electrode may bepartially coated with the polymer or other material to form a half-ringelectrode, quarter-ring electrode, or other partial-ring electrode.

ICD 14 may obtain sensed electrical signals corresponding withelectrical activity of heart 26 via a combination of sensing vectorsthat include individual electrodes or combinations of electrodes 28A,28B, and 30 and the housing electrode of ICD 14. For example, ICD 14 mayobtain electrical signals sensed using a sensing vector betweencombinations of electrodes 28A, 28B, and 30 with one another or obtainelectrical signals sensed using a sensing vector between any one or moreof electrodes 28A, 28B, and 30 and the conductive housing electrode ofICD 14. In some instances, ICD 14 may even obtain sensed electricalsignals using a sensing vector that includes one or both defibrillationelectrode segments 24A or 24B such as between each other or incombination with one or more of electrodes 28A, 28B, and 30, and/or thehousing electrode of ICD 14.

ICD 14 analyzes the sensed electrical signals obtained from one or moreof the sensing vectors of lead 16 to monitor for tachyarrhythmia, suchas ventricular tachycardia (VT) or ventricular fibrillation (VF). ICD 14may analyze the heart rate and/or morphology of the sensed electricalsignals to monitor for tachyarrhythmia in accordance with any of anumber of techniques known in the art. One example technique fordetecting tachyarrhythmia is described in U.S. Pat. No. 7,761,150 toGhanem et al., entitled “METHOD AND APPARATUS FOR DETECTING ARRHYTHMIASIN A MEDICAL DEVICE.” The entire content of the tachyarrhythmiadetection algorithm described in Ghanem et al. are incorporated byreference herein in its entirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting tachycardia (e.g., VT or VF). In response to detecting thetachycardia, ICD 14 may deliver one or more cardioversion ordefibrillation shocks via defibrillation electrode segments 24 of lead16. ICD 14 may deliver the cardioversion or defibrillation shocks usingeither, some or all of the electrode segments 24 individually ortogether as a cathode (or anode) and with the housing electrode as ananode (or cathode). ICD 14 may generate and deliver electricalstimulation therapy other than cardioversion or defibrillation shocks,including anti-tachycardia pacing (ATP), post-shock pacing, bradycardiapacing, high rate pacing for VF induction, and/or entrainment pacingpulses before a T-shock for VF induction using a therapy vector formedfrom one or more any of a variety of electrode vectors that include oneor more of the electrode segments 24 and/or electrodes 28A and/or 28Band/or 30, and/or the housing electrode of ICD 14. For example, ICD 14may deliver pacing pulses via an electrode vector in which one or moreof electrodes 28A, 28B, or 30, or electrode segments 24A and 24B(individually or collectively) is a cathode and the housing electrode isan anode or vice versa. In another example, ICD 14 may deliver pacingpulses via an electrode vector formed from various pairs or multipleconfigurations of electrodes 28A, 28B, 30, and/or electrode segments 24Aand 24B (individually or collectively), e.g., one or more of theelectrodes serve as a cathode and another one or more of the electrodesserve as an anode. In yet another example, ICD 14 may deliver pacingpulses via an electrode vector between any combination of electrodes28A, 28B, 30, and/or electrode segments 24A and 24B (individually orcollectively) concurrently used as a cathode (or anode) and the housingelectrode of ICD 14 as an anode or vice versa, as is the case inmulti-site or multi-point pacing. In a further example, ICD 14 may pacefrom one or more of electrodes 28A, 28B, and 30 to an individualdefibrillation electrode segment (e.g., 24A or 24B) when the segments 24are electrically coupled to separate conductors or from one or more ofelectrodes 28A, 28B, and 30 to the overall defibrillation electrodeformed by the combination of segments 24A and 24B when segments areelectrically coupled to a single conductor within lead body 16 or thesegments 24 are tied/jumpered together. Such an electrode vector maylimit extracardiac stimulation, improve pacing performance, allow forselection of another vector based on anatomy, or provide otherbenefit(s).

The examples illustrated in FIGS. 1A and 1B are exemplary in nature andshould not be considered limiting of the techniques described in thisdisclosure. In other examples, ICD 14 and lead 16 may be implanted atother locations. For example, ICD 14 may be implanted in a subcutaneouspocket in the right pectoral region. In this example, defibrillationlead 16 may extend subcutaneously from the device toward the manubriumof sternum 22 and bend or turn and extend inferior from the manubrium tothe desired location subcutaneously or substernally. In yet anotherexample, ICD 14 may be placed abdominally or intrathoracically. Lead 16may be implanted in other extravascular or extracardiac locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

In the example illustrated in FIG. 1, system 10 is an ICD system thatprovides cardioversion/defibrillation and, in some instances, pacingtherapy. However, these techniques may be applicable to other cardiacsystems, including cardiac resynchronization therapy defibrillator(CRT-D) systems or other cardiac stimulation therapies, or combinationsthereof. For example, ICD 14 may be configured to provide electricalstimulation pulses to stimulate nerves, skeletal muscles, diaphragmaticmuscles, e.g., for various neuro-cardiac applications and/or for sleepapnea or respiration therapy. As another example, lead 16 may be placedfurther superior such that at least one of the defibrillation electrodesegments 24 is placed substantially over the atrium of heart 26 toprovide a shock or pulse to the atrium to terminate atrial fibrillation(AF). In still other examples, defibrillation lead 16 may include asecond defibrillation electrode (e.g., second elongated coil electrode)near a proximal end of lead 16 or near a middle portion of lead 16.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted with anotherexample ICD system 110. FIG. 2A is a front view of patient 12 implantedwith ICD system 110. FIG. 2B is a side view of patient 12 implanted withICD system 110. FIG. 2C is a transverse view of patient 12 implantedwith ICD system 110. ICD system 110 can include one or more of thestructure and/or functionality of system 10 of FIGS. 1A-1B (and viceversa). ICD system 110 of FIGS. 2A-2C is illustrated with lead 16 forpurposes of illustration, but may be utilized with any of lead 16 or anyof the other leads described in this disclosure. Repetitive descriptionof like numbered elements described in other embodiments is omitted forsake of brevity.

ICD system 110 conforms substantially to ICD system 10 of FIGS. 1A-1B,except defibrillation lead 16 of system 110 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously from ICD 14 toward xiphoid process 20, and at a locationnear xiphoid process 20 bends or turns and extends superiorunderneath/below sternum 22 within anterior mediastinum 36. Anteriormediastinum 36 may be viewed as being bounded laterally by pleurae 39,posteriorly by pericardium 38, and anteriorly by sternum 22. In someinstances, the anterior wall of anterior mediastinum 36 may also beformed by the transversus thoracis and one or more costal cartilages.Anterior mediastinum 36 includes a quantity of loose connective tissue(such as areolar tissue), some lymph vessels, lymph glands, substernalmusculature (e.g., transverse thoracic muscle), branches of the internalthoracic artery, and the internal thoracic vein. In one example, thedistal portion of lead 16 extends along the posterior side of sternum 22substantially within the loose connective tissue and/or substernalmusculature of anterior mediastinum 36. A lead implanted such that thedistal portion is substantially within anterior mediastinum 36 will bereferred to herein as a substernal lead. Also, electrical stimulation,such as pacing, cardioversion or defibrillation, provided by lead 16implanted substantially within anterior mediastinum 36 will be referredto herein as substernal electrical stimulation, substernal pacing,substernal cardioversion, or substernal defibrillation.

The distal portion of lead 16 is described herein as being implantedsubstantially within anterior mediastinum 36. Thus, points along thedistal portion of lead 16 may extend out of anterior mediastinum 36, butthe majority of the distal portion is within anterior mediastinum 36. Inother embodiments, the distal portion of lead 16 may be implanted inother extracardiac, non-vascular, extra-pericardial locations, includingthe gap, tissue, or other anatomical features around the perimeter ofand adjacent to, but not attached to, the pericardium or other portionof heart 26 and not above sternum 22 or ribcage. As such, lead 16 may beimplanted anywhere within the “substernal space” defined by theundersurface between the sternum and/or ribcage and the body cavity butnot including the pericardium or other portion of heart 26. Thesubsternal space may alternatively be referred to by the terms“retrosternal space” or “mediastinum” or “infrasternal” as is known tothose skilled in the art and includes the anterior mediastinum 36. Thesubsternal space may also include the anatomical region described inBaudoin, Y. P., et al., entitled “The superior epigastric artery doesnot pass through Larrey's space (trigonum sternocostale),” Surg. Radiol.Anat. 25.3-4 (2003): 259-62. In other words, the distal portion of lead16 may be implanted in the region around the outer surface of heart 26,but not attached to heart 26. Moreover, in some instances, substernalspace may include inside the pleural membrane.

The distal portion of lead 16 may be implanted substantially withinanterior mediastinum 36 such that electrodes 28A and 28B are locatednear a ventricle of heart 26. To achieve improved sensing and/or pacing,it is desirable to have the pace/sense electrodes located substantiallyover the chamber of heart 26 that is being paced and/or sensed. Forinstance, lead 16 may be implanted within anterior mediastinum 36 suchthat electrodes 28A and 28B are located over a cardiac silhouette of oneor both ventricles as observed via an AP fluoroscopic view of heart 26.In other words, lead 16 may be implanted such that one or both of aunipolar pacing/sensing vector from electrode 28A or 28B to a housingelectrode of ICD 14 are substantially across the ventricles of heart 26.In another example, the spacing between electrodes 28A and 28B as wellas the placement of lead 16 may be such that a bipolar pacing vectorbetween electrodes 28A and 28B is centered or otherwise located over theventricle.

Likewise, to achieve improved defibrillation therapy, it is desirable tohave the defibrillation electrode segments 24A and 24B locatedsubstantially over the chamber of heart 26 to which the defibrillationor cardioversion shock is being applied, e.g., over a cardiac silhouetteof the ventricle as observed via an AP fluoroscopic view of heart 26. Inconventional subcutaneous lead designs, it is only possible to positioneither the defibrillation electrode or the sense electrode over therelevant chamber, but not both. Thus, not only are electrodes 28A and28B located over the ventricle, but due to the layout of the electrodeson the lead 16, defibrillation electrode segments 24A and 24B are alsosubstantially located over the cardiac silhouette of the ventricle asobserved via an AP fluoroscopic view of heart 26. In this manner, lead16 is designed to provide desirable electrode positioning for bothdefibrillation and pacing/sensing concurrently.

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallyenough such that all or a portion of lead 16 is underneath/below theribcage in addition to or instead of sternum 22.

Placing lead 16 in the substernal space may provide a number ofadvantages. For example, placing lead 16 in the substernal space maysignificantly reduce the amount of energy that needs to be delivered todefibrillate heart 26 compared to energy required to defibrillate theheart when the electrodes of the lead are placed subcutaneously. In someinstances, ICD 14 may generate and deliver cardioversion ordefibrillation shocks having energies of less than 65 Joules (J), lessthan 60 J, between 35-60 J, and in some cases possibly less than 35 J.As such, placing defibrillation lead 16 within the substernal space,e.g., with the distal portion substantially within anterior mediastinum36, may result in reduced energy consumption and, in turn, smallerdevices and/or devices having increased longevity compared to devicesused in conjunction with leads in which the electrodes are placedsubcutaneously above the ribcage and/or sternum.

Another advantage of placing lead 16 in the substernal space is thatpacing, such as anti-tachycardia pacing (ATP), post-shock pacing and, insome cases, bradycardia pacing, may be provided by system 110. Forexample, ICD 14 may deliver one or more sequences of ATP in an attemptto terminate a detected VT without delivering a defibrillation shock.The pacing (whether ATP, post-shock pacing, or bradycardia pacing) maybe delivered via any of the electrode vectors described above withrespect to FIGS. 1A and 1B. For example, pacing may be delivered via oneor more electrode vectors of lead 16, e.g., unipolar electrode vector,bipolar electrode vector (true or integrated) or multipolar electrodevector, formed using any of the following: electrodes 28A, 28B, and 30,the housing electrode of ICD 14, and/or defibrillation electrodesegments 24A and 24B collectively or individually. If the one or moresequences of ATP are not successful, it is determined that ATP is notdesired (e.g., in the case of VF), or ICD 14 is not configured todeliver ATP, ICD 14 may deliver one or more cardioversion ordefibrillation shocks via defibrillation electrode segments 24A or 24Bof lead 16. ICD 14 may deliver the cardioversion or defibrillationshocks using either of the electrode segments 24A and 24B (or anyadditional electrode segments 24 not depicted) individually or togethercollectively. ICD 14 may generate and deliver electrical stimulationtherapy other than ATP, cardioversion or defibrillation shocks,including post-shock pacing, bradycardia pacing, high rate pacing for VFinduction, entrainment pacing pulses before a T-shock for VF inductionand/or to sense low voltage signals and/or other electrical stimulationtherapy using a therapy vector formed from one or more of electrodes28A, 28B, 30, and/or electrode segments 24A and 24B (individually orcollectively), and/or the housing electrode.

In one example, ICD 14 may deliver pacing (e.g., ATP or post-shockpacing) using an electrode vector that includes one or bothdefibrillation electrode segments 24A and/or 24B. The electrode vectorused for pacing may, for example, include electrode segment 24A as ancathode (or anode) and one of electrode segment 24B and/or electrodes28A, 28B, 30, or the housing of ICD as the anode (or cathode) or includesegment 24B as an cathode (or anode) and one of electrode segment 24Aand/or electrodes 28A, 28B, 30 or the housing of ICD as the anode (orcathode), or include segments 24A and 24B together as a common cathode(or anode) and one electrodes 28A, 28B, 30, or the housing of ICD as theanode (or cathode). If high voltage therapy is necessary, ICD 14 maydeliver a cardioversion/defibrillation shock (or multiple shocks) usingboth of electrode segments 24A or 24B concurrently as the cathode andthe housing electrode of ICD 14 as the anode.

ICD 14 may also generate and deliver electrical stimulation signals forinducing VF, e.g., high rate pacing pulses and/or entrainment pacingpulses preceding a T-shock. In one example, ICD 14 may deliver high ratepacing pulses using an electrode vector between defibrillation electrodesegments 24A and 24B (e.g., one of segments 24 function as an anode andone of segments functioning as a cathode). In another example, ICD 32may deliver a plurality of entrainment pacing pulses (e.g., 3-5 pulses)using an electrode vector between defibrillation electrode segments 24Aand 24B and then deliver a shock adjacent a T-wave using defibrillationelectrode segments 24A and 24B collectively as a cathode and a housingelectrode of ICD 14 as an anode.

FIG. 3 is a conceptual diagram illustrating a distal portion of anotherexample implantable electrical lead 40 with improved pacing and/orsensing capability for use in extracardiac, extravascular, non-vascular,and/or extra-pericardial applications. Lead 40 can include one or moreof the structure and/or functionality of lead 16 of FIGS. 1A-1B andFIGS. 2A-2C (and vice versa). Repetitive description of like numberedelements described in other embodiments is omitted for sake of brevity.Lead 40 may be used in place of lead 16 in ICD system 10 of FIGS. 1A and1B or ICD system 110 of FIGS. 2A-2C.

Lead 40 conforms substantially with lead 16 of FIGS. 1A and 1B, butinstead of having ring electrodes 28A and 28B located betweendefibrillation electrode segments 24A and 24B, lead 40 includes apace/sense coil electrode 42 between defibrillation electrode segments24A and 24B and a pace/sense ring electrode 28 proximal todefibrillation electrode segment 24B. Such a configuration may increasethe surface area of the pace/sense electrode located over theventricle(s). However, pace/sense coil electrode 42 may be positionedelsewhere along lead body 40, including distal to defibrillationelectrode segment 24A, proximal to defibrillation electrode segment 24Bor between additional defibrillation electrode segments 24 not depicted.Lead 40 may thus contain more than one pace/sense coil electrode 42along its length, and pace/sense coil electrodes 42 may be the same ordifferent from each other in terms of size, shape, type or material.Moreover a portion of coil 42 may be partially coated or masked with apolymer (e.g., polyurethane) or another coating material (e.g., tantalumpentoxide) on one side or in different regions so as to direct thepacing signal toward heart 26 and not outward toward skeletal muscle.

Defibrillation electrode segments 24A and/or 24B may have any of thelengths (individual or total) described above with respect to FIGS. 1Aand 1B and may be substantially the same length or different lengths.Additionally, defibrillation electrode segments 24A and 24B may bespaced apart from one another by any of the distances described abovewith respect to FIGS. 1A and 1B. Pace/sense coil electrode 42 may have alength of between approximately greater than or equal to 0.5 and lessthan or equal to 3 cm. Pace/sense coil electrode 42 is also spaced apartfrom defibrillation electrode segments 24A and 24B (e.g., the proximalend of electrode 42 is spaced apart from the distal end of electrodesegment 24B and the distal end of electrode 42 is spaced apart from theproximal end of electrode segment 24A) by the distances described abovewith respect to FIGS. 1A and 1B. As such, the length of pace/sense coilelectrode 42 may be dependent upon the spacing between defibrillationelectrode segments 24A and 24B. In one example, defibrillation electrodesegments 24A and 24B may each have lengths approximately equal to 4 cmand be spaced apart by a distance greater than 1 cm. In this case, thepace/sense coil electrode 42 may have a length of approximately 1 cm.However, other spacings and lengths greater than or less than 1 cm maybe used, including the ranges provided above with respect to FIGS. 1Aand 1B.

As described above, lead 40 of FIG. 3 includes a tip electrode 30 distalto defibrillation electrode segment 24A and a ring electrode 28 proximalto defibrillation electrode segment 24B. Tip electrode 30 may be spacedapart from defibrillation electrode segment 24A and ring electrode 28may be spaced apart from defibrillation electrode segment 24B by any ofthe distances described above with respect to FIGS. 1A and 1B. In someinstances, the distal portion of lead 40 from the distal end of lead 40to the proximal side of the most proximal electrode (e.g., electrode 28in the example of FIG. 3) may be less than or equal 15 cm and, morepreferably, less than or equal to 13 cm and even more preferably lessthan or equal to 10 cm.

Although illustrated as a ring electrode and hemispherical electrode,respectively, electrodes 28 and 30 may comprise any of a number ofdifferent types of electrodes, including ring electrodes, short coilelectrodes, paddle electrodes, hemispherical electrodes, directionalelectrodes, segmented electrodes, or the like, and may be positioned atany position along the distal portion of lead 40. Further, electrodes 28and 30 may be of similar type, shape, size and material or may differfrom each other.

Moreover, in other embodiments lead 40 may not include one or both ofelectrodes 28 and/or 30. For example, lead 40 may include coil electrode42 as the only pace/sense electrode on lead 40 and not include either ofelectrodes 28 or 30. In another example, lead 40 may include distalelectrode 30 and coil electrode 42 as pace/sense electrodes and notinclude proximal electrode 28. In a further example, lead 40 may includeproximal electrode 28 and coil electrode 42 as pace/sense electrodes andnot include distal electrode 30. In all of the examples in thisparagraph, lead 40 includes defibrillation electrode segments 24A and24B (and additional defibrillation electrode segments 24, not depicted),which may be used for pacing and sensing as well.

ICD 14 may be configured to sense and deliver pacing and/orcardioversion/defibrillation using any combination of electrode segments24A and/or 24B, and/or electrodes 28, 30, 42, and/or the housingelectrode. ICD may deliver electrical stimulation and/or sense using anyelectrode vector that includes defibrillation electrode segments 24A and24B (individually or collectively), and/or electrodes 28, 30, and/or 42,and/or the housing electrode of ICD 14. For example, ICD 14 may deliverpacing pulses via an electrode vector in which one or more of electrodes28, 30, 42, or electrode segments 24A and/or 24B (individually orcollectively) is a cathode and the housing electrode of ICD 14 is ananode or possibly any combination or cathode and anode configurations.In another example, ICD 14 may deliver pacing pulses via an electrodevector formed from various pairs or multiple configurations ofelectrodes 28, 30, 42, or electrode segments 24A and/or 24B(individually or collectively), e.g., one or more of the electrodesand/or segments serves as a cathode and another one or more of theelectrodes and/or segments serves as an anode. In yet another example,ICD 14 may deliver pacing pulses via an electrode vector between anycombination of electrodes 28, 30, 42, and/or electrode segments 24Aand/or 24B (individually or collectively) that are concurrently used asa cathode and the housing electrode of ICD 14 is used as an anode orvice versa, as is the case in multi-site or multi-point pacing.

Lead 40 may be implanted in any of the locations described above withrespect to FIGS. 1A-1B and FIGS. 2A-2C, e.g., such that electrode 42and/or defibrillation electrode segment(s) 24 are substantially over theventricular surface of the cardiac silhouette.

FIG. 4 is a conceptual diagram illustrating a distal portion of anotherexample implantable electrical lead 80 with improved pacing and/orsensing capability for use in extracardiac, extravascular, non-vascular,and/or extra-pericardial applications. Lead 80 can include one or moreof the structure and/or functionality of lead 16 of FIGS. 1A-1B andFIGS. 2A-2C (and vice versa) and/or lead 40 of FIG. 3 (and vice versa).Repetitive description of like numbered elements described in otherembodiments is omitted for sake of brevity. Lead 80 may be used in placeof lead 16 in ICD system 10 of FIGS. 1A and 1B or ICD system 110 ofFIGS. 2A-2C.

Lead 80 conforms substantially with lead 16 of FIGS. 1A and 1B, butinstead of having ring electrodes 28A and 28B located betweendefibrillation electrode segments 24A and 24B, lead 80 includes only asingle pace/sense electrode 28 between defibrillation electrode segments24A and 24B and different defibrillation electrode segment lengths.

Defibrillation electrode segments 24A and/or 24B of FIG. 4 are ofdifferent lengths. In particular, defibrillation electrode segment 24Ais longer than defibrillation electrode segment 24B. In other instances,however, defibrillation electrode segment 24B may be longer thandefibrillation electrode segment 24A or the two may have substantiallythe same length. The lengths (individual or total) of segments 24A and24B may be the same as described above with respect to FIGS. 1A and 1B.Additionally, defibrillation electrode segments 24A and 24B may bespaced apart from one another by any of the distances described abovewith respect to FIGS. 1A and 1B.

Lead 80 includes a tip electrode 30 distal to defibrillation electrodesegment 24A and ring electrode 28 between defibrillation electrodesegment 24A and 24B. Tip electrode 30 may be spaced apart fromdefibrillation electrode segment 24A and ring electrode 28 may be spacedapart from defibrillation electrode segments 24A and 24B by any of thedistances described above with respect to FIGS. 1A and 1B. In someinstances, the distal portion of lead 80 from the distal end of lead 80to the proximal side of the most proximal electrode (e.g., electrodesegment 24B in the example of FIG. 4) may be less than or equal 15 cmand, more preferably, less than or equal to 13 cm and even morepreferably less than or equal to 10 cm.

Although illustrated as a ring electrode and hemispherical electrode,respectively, electrodes 28 and 30 may comprise any of a number ofdifferent types of electrodes, including ring electrodes, short coilelectrodes, paddle electrodes, hemispherical electrodes, directionalelectrodes, segmented electrodes, or the like, and may be positioned atany position along the distal portion of lead 80. Further, electrodes 28and 30 may be of similar type, shape, size and material or may differfrom each other.

Moreover, in other embodiments lead 80 may not include one or both ofelectrodes 28 and/or 30. For example, lead 80 may include electrode 28between defibrillation segments 24A and 24B but not include electrode30. In another example, lead 80 may include electrode 28 betweendefibrillation segments 24A and 24B and include another pace/senseelectrode proximal to defibrillation electrode segment 24B in additionto or instead of pace/sense electrode 30. In other instances, lead 80may include more than one pace/sense electrode distal to electrodesegment 24A and/or more than one pace/sense electrode betweendefibrillation electrode segments 24A and 24B and/or more than onepace/sense electrode proximal to defibrillation electrode segment 24B.In all of the examples in this paragraph, lead 80 includesdefibrillation electrode segments 24A and 24B, which may be used forpacing and sensing as well.

ICD 14 may be configured to sense and/or deliver pacing and/orcardioversion/defibrillation using any combination of electrode segments24A and/or 24B, electrodes 28, 30, and the housing electrode. ICD maydeliver electrical stimulation and/or sense using any electrode vectorthat includes defibrillation electrode segments 24A and 24B(individually or collectively), and/or electrodes 28 and/or 30, and/orthe housing electrode of ICD 14. For example, ICD 14 may deliver pacingpulses via an electrode vector in which one of electrodes 28, 30, orelectrode segments 24A and/or 24B (individually or collectively) is acathode and the housing electrode of ICD 14 is an anode or vice versa.In another example, ICD 14 may deliver pacing pulses via an electrodevector formed from various pairs or multiple configurations ofelectrodes 28, 30, and/or electrode segments 24A and/or 24B, as well asadditional electrode segments 24 not depicted, (individually orcollectively), e.g., one or more of the electrodes and/or segmentsserves as a cathode and another one or more of the electrodes and/orsegments serves as an anode. In yet another example, ICD 14 may deliverpacing pulses via an electrode vector between any combination ofelectrodes 28, 30, and/or electrode segments 24A and/or 24B(individually or collectively) that are concurrently used as a cathodeand the housing electrode of ICD 14 is used as an anode or vice versa,as is the case in multi-site or multi-point pacing.

Lead 80 may be implanted in any of the locations described above withrespect to FIGS. 1A-1B and FIGS. 2A-2C, e.g., such that electrode 28and/or defibrillation electrode segments 24A and/or 24B aresubstantially over the ventricular surface of the cardiac silhouette.

FIG. 5 is a conceptual diagram illustrating a distal portion of anotherexample implantable electrical lead 90 with improved pacing and/orsensing capability for use in extracardiac, extravascular, non-vascular,and/or extra-pericardial applications. Lead 90 can include one or moreof the structure and/or functionality of lead 16 of FIGS. 1A-1B andFIGS. 2A-2C (and vice versa), lead 40 of FIG. 3 (and vice versa), and/orlead 80 of FIG. 4 (and vice/versa). Repetitive description of likenumbered elements described in other embodiments is omitted for sake ofbrevity. Lead 90 may be used in place of lead 16 in ICD system 10 ofFIGS. 1A and 1B or ICD system 110 of FIGS. 2A-2C.

Lead 90 includes defibrillation electrode segments 24A and 24B, a firstpace/sense electrode 28A disposed between defibrillation electrodesegments 24A and 24B, and a second pace/sense electrode 28B disposedproximal to defibrillation electrode segment 24B. Defibrillationelectrode segments 24A and/or 24B may have any of the lengths(individual or total) described above with respect to FIGS. 1-4 and maybe substantially the same length or different lengths. Additionally,defibrillation electrode segments 24A and 24B may be spaced apart fromone another by any of the distances described above with respect toFIGS. 1-4.

Pace/sense electrodes 28 are spaced apart from defibrillation electrodesegments 24A and 24B by the distances described above with respect toFIGS. 1-4. In some instances, the distal portion of lead 90 from thedistal end of lead 90 to the proximal side of the most proximalelectrode (e.g., electrode 28B in the example of FIG. 5) may be lessthan or equal 15 cm and, more preferably, less than or equal to 13 cmand even more preferably less than or equal to 10 cm. Althoughillustrated as ring electrodes, electrodes 28 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, paddle electrodes, hemispherical electrodes,directional electrodes, segmented electrodes, or the like, and may bepositioned at any position along the distal portion of lead 90. Further,electrodes 28 may be of similar type, shape, size and material or maydiffer from each other.

Moreover, in other embodiments, lead 90 may not include one or both ofelectrodes 28A and/or 28B. For example, lead 90 may include electrode28A between defibrillation segments 24A and 24B but not includeelectrode 28B. In another example, electrode 28B of lead 90 may belocated distal to defibrillation electrode segment 24A. In a furtherexample, in addition to electrodes 28A and 28B, lead 90 may include athird pace/sense electrode located distal to defibrillation electrodesegment 28A.

ICD 14 may deliver electrical stimulation and/or sense electricalsignals using any electrode vector that includes defibrillationelectrode segments 24A and 24B (individually or collectively), and/orelectrodes 28A and/or 28B, and/or the housing electrode of ICD 14. Forexample, ICD 14 may deliver pacing pulses via an electrode vector inwhich one of electrodes 28A, 28B, or electrode segments 24A and/or 24B(individually or collectively) is a cathode and the housing electrode ofICD 14 is an anode or vice versa. In another example, ICD 14 may deliverpacing pulses via an electrode vector formed from various pairs ormultiple configurations of electrodes 28A, 28B, and/or electrodesegments 24A and/or 24B, as well as additional electrode segments 24 notdepicted, (individually or collectively), e.g., one or more of theelectrodes and/or segments serves as a cathode and another one or moreof the electrodes and/or segments serves as an anode. In yet anotherexample, ICD 14 may deliver pacing pulses via an electrode vectorbetween any combination of electrodes 28A, 28B, and/or electrodesegments 24A and/or 24B (individually or collectively) that areconcurrently used as a cathode and the housing electrode of ICD 14 isused as an anode or vice versa, as is the case in multi-site ormulti-point pacing. Many of these vectors are described in more detailabove with respect to FIGS. 1-4.

Lead 90 may be implanted in any of the locations described above withrespect to FIGS. 1A-1B and FIGS. 2A-2C, e.g., such that electrode 28Aand/or 28B and/or defibrillation electrode segments 24A and/or 24B aresubstantially over the ventricular surface of the cardiac silhouette.

FIG. 6 is a functional block diagram of an example configuration ofelectronic components of an example ICD 14. ICD 14 includes a controlmodule 60, sensing module 62, therapy module 64, communication module68, and memory 70. The electronic components may receive power from apower source 66, which may be a rechargeable or non-rechargeablebattery. In other embodiments, ICD 14 may include more or fewerelectronic components. The described modules may be implemented togetheron a common hardware component or separately as discrete butinteroperable hardware or software components. Depiction of differentfeatures as modules is intended to highlight different functionalaspects and does not necessarily imply that such modules must berealized by separate hardware or software components. Rather,functionality associated with one or more modules may be performed byseparate hardware or software components, or integrated within common orseparate hardware or software components. FIG. 6 will be described inthe context of ICD 14 being coupled to lead 16 for exemplary purposesonly. However, ICD 14 may be coupled to other leads, such as lead 40and/or lead 80 described herein, and thus other electrodes, such aselectrodes 42.

Sensing module 62 is electrically coupled to some or all of electrodes24 (or separately to segments 24A and/or 24B), 28A, 28B, and 30 via theconductors of lead 16 and one or more electrical feedthroughs, or to thehousing electrode via conductors internal to the housing of ICD 14.Sensing module 62 is configured to obtain signals sensed via one or morecombinations of electrodes 24 (or segments 24A and/or 24B), 28A, 28B,and 30 and the housing electrode of ICD 14 and process the obtainedsignals.

The components of sensing module 62 may be analog components, digitalcomponents or a combination thereof. Sensing module 62 may, for example,include one or more sense amplifiers, filters, rectifiers, thresholddetectors, analog-to-digital converters (ADCs) or the like. Sensingmodule 62 may convert the sensed signals to digital form and provide thedigital signals to control module 60 for processing or analysis. Forexample, sensing module 62 may amplify signals from the sensingelectrodes and convert the amplified signals to multi-bit digitalsignals by an ADC. Sensing module 62 may also compare processed signalsto a threshold to detect the existence of atrial or ventriculardepolarizations (e.g., P- or R-waves) and indicate the existence of theatrial depolarization (e.g., P-waves) or ventricular depolarizations(e.g., R-waves) to control module 60.

Control module 60 may process the signals from sensing module 62 tomonitor electrical activity of heart 26 of patient 12. Control module 60may store signals obtained by sensing module 62 as well as any generatedEGM waveforms, marker channel data or other data derived based on thesensed signals in memory 70. Control module 60 may analyze the EGMwaveforms and/or marker channel data to detect cardiac events (e.g.,tachycardia). In response to detecting the cardiac event, control module60 may control therapy module 64 to deliver the desired therapy to treatthe cardiac event, e.g., defibrillation shock, cardioversion shock, ATP,post-shock pacing, or bradycardia pacing.

Therapy module 64 is configured to generate and deliver electricalstimulation therapy to heart 26. Therapy module 64 may include one ormore pulse generators, capacitors, and/or other components capable ofgenerating and/or storing energy to deliver as pacing therapy,defibrillation therapy, cardioversion therapy, cardiac resynchronizationtherapy, other therapy or a combination of therapies. In some instances,therapy module 64 may include a first set of components configured toprovide pacing therapy and a second set of components configured toprovide defibrillation therapy. In other instances, therapy module 64may utilize the same set of components to provide both pacing anddefibrillation therapy. In still other instances, therapy module 64 mayshare some of the defibrillation and pacing therapy components whileusing other components solely for defibrillation or pacing.

Control module 60 may control therapy module 64 to deliver the generatedtherapy to heart 26 via one or more combinations of electrodes 24 (orseparately to segments 24A and/or 24B), 28A, 28B, and 30 of lead 16 andthe housing electrode of ICD 14 according to one or more therapyprograms, which may be stored in memory 70. In instances in whichcontrol module 60 is coupled to a different lead, e.g., lead 40, 80, or90, other electrodes may be utilized, such as electrodes 28 and 42.Control module 60 controls therapy module 64 to generate electricalstimulation therapy with the amplitudes, pulse widths, timing,frequencies, electrode combinations or electrode configurationsspecified by a selected therapy program.

Therapy module 64 may include a switch module to select which of theavailable electrodes are used to deliver the therapy. The switch modulemay include a switch array, switch matrix, multiplexer, or any othertype of switching device suitable to selectively couple electrodes totherapy module 64. Control module 60 may select the electrodes tofunction as therapy electrodes, or the therapy vector, via the switchmodule within therapy module 64. In instances in which defibrillationsegments 24A and 24B are each coupled to separate conductors, controlmodule 60 may be configured to selectively couple therapy module 64 toeither one of segments 24A or 24B individually or couple to both of thesegments 24A and 24B concurrently. In some instances, the same switchmodule may be used by both therapy module 64 and sensing module 62. Inother instances, each of sensing module 62 and therapy module 64 mayhave separate switch modules.

In the case of pacing therapy being provided, e.g., ATP, post-shockpacing, and/or bradycardia pacing provided via electrodes 28A, 28B, 30,and/or defibrillation electrode segments 24A or 24B of lead 16. In oneexample, therapy module 64 may deliver pacing (e.g., ATP or post-shockpacing) using an electrode vector that includes one or bothdefibrillation electrode segments 24A and/or 24B. The electrode vectorused for pacing may be segment 24A as an anode (or cathode) and one ofelectrodes 24B, 28A, 28B, 30 or the housing of ICD as the cathode (oranode) or segment 24B as an anode (or cathode) and one of electrodes24A, 28A, 28B, 30 or the housing of ICD as the cathode (or anode). Ifnecessary, therapy module 64 may generate and deliver acardioversion/defibrillation shock (or shocks) using one or both ofelectrode segments 24A or 24B concurrently as a cathode and the housingelectrode of ICD 14 as an anode.

Control module 60 controls therapy module 64 to generate and deliverpacing pulses with any of a number of shapes, amplitudes, pulse widths,or other characteristic to capture heart 26. For example, the pacingpulses may be monophasic, biphasic, or multiphasic (e.g., more than twophases). The pacing thresholds of heart 26 when delivering pacing pulsesfrom the substernal space, e.g., from electrodes 28A, 28B, and/or 30and/or electrode segments 24 substantially within anterior mediastinum36, may depend upon a number of factors, including location, type, size,orientation, and/or spacing of electrodes 28A, 28B, and 30 and/orelectrode segments 24, location of ICD 14 relative to electrodes 28A,28B, and 30 and/or electrode segments 24, physical abnormalities ofheart 26 (e.g., pericardial adhesions or myocardial infarctions), orother factor(s).

The increased distance from electrodes 28A, 28B, and 30 and/or electrodesegments 24 of lead 16 to the heart tissue may result in heart 26 havingincreased pacing thresholds compared to transvenous pacing thresholds.To this end, therapy module 64 may be configured to generate and deliverpacing pulses having larger amplitudes and/or pulse widths thanconventionally required to obtain capture via leads implanted within theheart (e.g., transvenous leads) or leads attached directly to heart 26.In one example, therapy module 64 may generate and deliver pacing pulseshaving amplitudes of less than or equal to 8 volts and pulse widthsbetween 0.5-3.0 milliseconds and, in some instances up to 4milliseconds. In another example, therapy module 64 may generate anddeliver pacing pulses having amplitudes of between 5 and 10 volts andpulse widths between approximately 3.0 milliseconds and 10.0milliseconds. In another example, therapy module 64 may generate anddeliver pacing pulses having pulse widths between approximately 2.0milliseconds and 8.0 milliseconds. In a further example, therapy module64 may generate and deliver pacing pulses having pulse widths betweenapproximately 0.5 milliseconds and 20.0 milliseconds. In anotherexample, therapy module 64 may generate and deliver pacing pulses havingpulse widths between approximately 1.5 milliseconds and 20.0milliseconds.

Pacing pulses having longer pulse durations than conventionaltransvenous pacing pulses may result in lower energy consumption. Assuch, therapy module 64 may be configured to generate and deliver pacingpulses having pulse widths or durations of greater than two (2)milliseconds. In another example, therapy module 64 may be configured togenerate and deliver pacing pulses having pulse widths or durations ofbetween greater than two (2) milliseconds and less than or equal tothree (3) milliseconds. In another example, therapy module 64 may beconfigured to generate and deliver pacing pulses having pulse widths ordurations of greater than or equal to three (3) milliseconds. In anotherexample, therapy module 64 may be configured to generate and deliverpacing pulses having pulse widths or durations of greater than or equalto four (4) milliseconds. In another example, therapy module 64 may beconfigured to generate and deliver pacing pulses having pulse widths ordurations of greater than or equal to five (5) milliseconds. In anotherexample, therapy module 64 may be configured to generate and deliverpacing pulses having pulse widths or durations of greater than or equalto ten (10) milliseconds. In a further example, therapy module 64 may beconfigured to generate and deliver pacing pulses having pulse widthsbetween approximately 3-10 milliseconds. In a further example, therapymodule 64 may be configured to generate and deliver pacing pulses havingpulse widths between approximately 4-10 milliseconds. In a furtherexample, therapy module 64 may be configured to generate and deliverpacing pulses having pulse widths or durations of greater than or equalto fifteen (15) milliseconds. In yet another example, therapy module 64may be configured to generate and deliver pacing pulses having pulsewidths or durations of greater than or equal to twenty (20)milliseconds.

Depending on the pulse widths, ICD 14 may be configured to deliverpacing pulses having pulse amplitudes less than or equal to twenty (20)volts, deliver pacing pulses having pulse amplitudes less than or equalto ten (10) volts, deliver pacing pulses having pulse amplitudes lessthan or equal to five (5) volts, deliver pacing pulses having pulseamplitudes less than or equal to two and one-half (2.5) volts, deliverpacing pulses having pulse amplitudes less than or equal to one (1)volt. In other examples, the pacing pulse amplitudes may be greater than20 volts. Typically the lower amplitudes require longer pacing widths asillustrated in the experimental results. Reducing the amplitude ofpacing pulses delivered by ICD 14 reduces the likelihood ofextra-cardiac stimulation and lower consumed energy of power source 66.

For pacing therapy provided from the subcutaneous placement of lead 16above the sternum and/or ribcage, pacing amplitudes and pulse widths mayvary.

In the case of cardioversion or defibrillation therapy, e.g.,cardioversion or defibrillation shocks provided by defibrillationelectrode segments 24A and/or 24B (individually or together), controlmodule 60 controls therapy module 64 to generate cardioversion ordefibrillation shocks having any of a number of waveform properties,including leading-edge voltage, tilt, delivered energy, pulse phases,and the like. Therapy module 64 may, for instance, generate monophasic,biphasic or multiphasic waveforms. Additionally, therapy module 64 maygenerate cardioversion or defibrillation waveforms having differentamounts of energy. As with pacing, delivering cardioversion ordefibrillation shocks from the substernal space, e.g., from electrodesegment(s) 24 substantially within anterior mediastinum 36, may reducethe amount of energy that needs to be delivered to defibrillate heart26. When lead 16 is implanted in the substernal space, therapy module 64may generate and deliver cardioversion or defibrillation shocks havingenergies of less than 65 J, less than 60 J, between 40-50 J, between35-60 J, and in some instances less than 35 J. When lead 16 is implantedsubcutaneously, ICD 14 may generate and deliver cardioversion ordefibrillation shocks having energies around 65-80 J.

Therapy module 64 may also generate defibrillation waveforms havingdifferent tilts. In the case of a biphasic defibrillation waveform,therapy module 64 may use a 65/65 tilt, a 50/50 tilt, or othercombinations of tilt. The tilts on each phase of the biphasic ormultiphasic waveforms may be the same in some instances, e.g., 65/65tilt. However, in other instances, the tilts on each phase of thebiphasic or multiphasic waveforms may be different, e.g., 65 tilt on thefirst phase and 55 tilt on the second phase. The example deliveredenergies, leading-edge voltages, phases, tilts, and the like areprovided for example purposes only and should not be considered aslimiting of the types of waveform properties that may be utilized toprovide substernal defibrillation via defibrillation electrodesegment(s) 24.

Communication module 68 includes any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice, such as a clinician programmer, a patient monitoring device, orthe like. For example, communication module 68 may include appropriatemodulation, demodulation, frequency conversion, filtering, and amplifiercomponents for transmission and reception of data with the aid ofantenna 72. Antenna 72 may be located within connector block of ICD 14or within housing ICD 14.

The various modules of ICD 14 may include any one or more processors,controllers, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs), orequivalent discrete or integrated circuitry, including analog circuitry,digital circuitry, or logic circuitry. Memory 70 may includecomputer-readable instructions that, when executed by control module 60or other component of ICD 14, cause one or more components of ICD 14 toperform various functions attributed to those components in thisdisclosure. Memory 70 may include any volatile, non-volatile, magnetic,optical, or electrical media, such as a random access memory (RAM),read-only memory (ROM), non-volatile RAM (NVRAM), static non-volatileRAM (SRAM), electrically-erasable programmable ROM (EEPROM), flashmemory, or any other non-transitory computer-readable storage media.

Various examples have been described. These and other examples arewithin the scope of the following claims.

The invention claimed is:
 1. An implantable medical electrical leadcomprising: an elongated lead body having a distal portion and aproximal end; at least one pace/sense electrode located along the distalportion of the elongated lead body; a defibrillation electrode locatedalong the distal portion of the elongated lead body, wherein thedefibrillation electrode includes a first defibrillation electrodesegment and a second defibrillation electrode segment spaced proximal tothe first defibrillation electrode segment, and wherein the distalportion of the elongated lead body is arranged such that, when theimplantable medical electrical lead is implanted in a substernallocation in a patient, the defibrillation electrode is substantiallyover a surface of a cardiac silhouette of a heart of the patient asobserved via an anterior-posterior (AP) fluoroscopic view of the heart,and the at least one pace/sense electrode is over a surface of asilhouette of a ventricle of the heart as observed via the APfluoroscopic view of the heart.
 2. The implantable medical electricallead of claim 1, wherein when the implantable medical electrical lead isimplanted in the substernal location, the defibrillation electrode issubstantially over a center of the surface of the silhouette of theventricle as observed via the AP fluoroscopic view of the heart.
 3. Theimplantable medical electrical lead of claim 1, wherein the first andsecond defibrillation electrode segments are spaced apart from eachother by a distance of approximately 0.5 centimeters to 2 centimeters.4. The implantable medical electrical lead of claim 3, wherein the firstand second defibrillation electrode segments are spaced apart from eachother by a distance of approximately 20 millimeters.
 5. The implantablemedical electrical lead of claim 1, wherein the at least one pace/senseelectrode is located between the first and second defibrillationelectrode segments.
 6. The implantable medical electrical lead of claim1, wherein the at least one pace/sense electrode is located distal tothe first defibrillation electrode segment.
 7. The implantable medicalelectrical lead of claim 1, wherein the at least one pace/senseelectrode is located proximal to the second defibrillation electrodesegment.
 8. The implantable medical electrical lead of claim 1, whereinthe at least one pace/sense electrode comprises at least two pace/senseelectrodes, and wherein a first of the at least two pace/senseelectrodes is located between the first and second defibrillationelectrode segments and a second of the at least two pace/senseelectrodes is located proximal to the second defibrillation electrodesegment.
 9. The implantable medical electrical lead of claim 1, whereinthe at least one pace/sense electrode comprises at least two pace/senseelectrodes, and wherein a first of the at least two pace/senseelectrodes is located between the first and second defibrillationelectrode segments and a second of the at least two pace/senseelectrodes is located distal to the first defibrillation electrodesegment.
 10. The implantable medical electrical lead of claim 9,wherein, when the implantable medical electrical lead is implanted inthe substernal location in the patient, the second of the at least twopace/sense electrodes is located over a cardiac silhouette of an atriumof the heart as observed via the AP fluoroscopic view.
 11. Theimplantable medical electrical lead of claim 1, wherein the at least onepace/sense electrode and at least one of the first and seconddefibrillation electrode segments are both substantially over thesurface of the silhouette of the ventricle as observed via the APfluoroscopic view.
 12. The implantable medical electrical lead of claim11, wherein the second defibrillation electrode segment is substantiallyover the surface of the silhouette of the ventricle as observed via theAP fluoroscopic view.
 13. The implantable medical electrical lead ofclaim 12, wherein the at least one pace/sense electrode and the seconddefibrillation electrode segment are both substantially over a center ofa ventricle.
 14. The implantable medical electrical lead of claim 1,wherein the substernal location comprises a location substantiallywithin an anterior mediastinum of the patient, and wherein the distalportion of the elongated lead body is configured to be positioned in thelocation substantially within the anterior mediastinum of the patient.15. An extravascular implantable cardioverter-defibrillator systemcomprising: an implantable cardioverter-defibrillator (ICD) thatincludes a housing electrode and a therapy module configured to generateand deliver electrical stimulation therapy; and an extravascularimplantable medical electrical lead electrically coupled to the therapymodule, wherein the lead comprises: an elongated lead body having adistal portion and a proximal end; a defibrillation electrode locatedalong the distal portion of the elongated lead body, wherein thedefibrillation electrode includes a first defibrillation electrodesegment and a second defibrillation electrode segment spaced proximal tothe first defibrillation electrode segment; and at least one pace/senseelectrode located along the distal portion of the elongated lead body,wherein the distal portion of the elongated lead body is arranged suchthat, when the implantable medical electrical lead is implanted in asubsternal location in a patient, the defibrillation electrode issubstantially over a surface of a cardiac silhouette of a heart of thepatient as observed via the AP fluoroscopic view and the at least onepace/sense electrode is over a surface of a silhouette of a ventricle ofthe heart as observed via the AP fluoroscopic view.
 16. The system ofclaim 15, wherein the at least one pace/sense electrode comprises atleast two pace/sense electrodes.
 17. The system of claim 16, whereinwhen the implantable medical electrical lead is implanted, the at leasttwo pace/sense electrodes are substantially located over the surface ofthe silhouette of the ventricle as observed via the AP fluoroscopicview.
 18. The system of claim 15, wherein when the implantable medicalelectrical lead is implanted, a unipolar pacing vector from the at leastone pace/sense electrode to the housing electrode is substantiallyacross the ventricle.
 19. The system of claim 16, wherein when theimplantable medical electrical lead is implanted, the at least twopace/sense electrodes are substantially located over the surface of thesilhouette of the ventricle as observed via the AP fluoroscopic view,and wherein a bipolar pacing vector between two of the at least twopace/sense electrodes is substantially over the ventricle.
 20. Thesystem of claim 15, further comprising an electrical conductor extendingfrom the proximal end of the elongated lead body and electricallycoupling to the first defibrillation electrode segment and the seconddefibrillation electrode segment, and wherein the first and seconddefibrillation electrode segments form one of a common cathode and acommon anode of an electrode vector for delivery of electricalstimulation therapy.
 21. The system of claim 15, further comprising: afirst electrical conductor extending from the proximal end of theelongated lead body and electrically coupling to the firstdefibrillation electrode segment; and a second electrical conductorextending from the proximal end of the elongated lead body andelectrically coupling to the second defibrillation electrode segment.22. The system of claim 15, wherein a length from a distal end of thelead to a proximal side of the most proximal electrode is less than orequal 15 cm.
 23. The system of claim 15, wherein the first and seconddefibrillation electrode segments are spaced apart from each other by adistance of approximately 20 millimeters.
 24. The system of claim 15,wherein the at least one pace/sense electrode is located between thefirst and second defibrillation electrode segments.
 25. The system ofclaim 15, wherein the at least one pace/sense electrode is locatedproximal to the second defibrillation electrode segment.
 26. The systemof claim 15, wherein the at least one pace/sense electrode and thesecond defibrillation electrode segment are both substantially over thesurface of the silhouette of the ventricle of the heart as observed viathe AP fluoroscopic view.
 27. The system of claim 15, wherein thesubsternal location comprises a location substantially within ananterior mediastinum of the patient, and wherein the distal portion ofthe elongated lead body is configured to be positioned in the locationsubstantially within the anterior mediastinum of the patient.
 28. Thesystem of claim 15, wherein the at least one pace/sense electrode of thelead comprises at least two pace/sense electrodes, and wherein a firstof the at least two pace/sense electrodes is located between the firstand second defibrillation electrode segments and a second of at leasttwo pace/sense electrodes is located distal to the first defibrillationelectrode segment.
 29. The system of claim 28, wherein, when theimplantable medical electrical lead is implanted in the substernallocation in the patient, the second of the at least two pace/senseelectrodes is located over a cardiac silhouette of an atrium of theheart as observed via the AP fluoroscopic view.